Pressure is mounting on the Democratic Unionist Party to reveal who bankrolled its expensive UK-wide pro-Brexit campaign.

More than a quarter of a million pounds is believed to have been spent by the DUP to promote its leave Europe message.

As part of that campaign, the party paid for an expensive four page Vote To Leave EU advertisement in the British Metro newspaper. The paper is available in London and other cities but not in Northern Ireland.

Questions have arisen over how the DUP raised the cash for a costly Brexit campaign.

DUP leader Arlene Foster has admitted that the party received donations after registering as a leave the European Union campaigner.

However, when pushed for details about the donors during a UTV pre-election debate in Northern Ireland on Thursday night she would only say: "From an organisation in England that wants to see the Union kept and make sure we can have a United Kingdom."

Donations to political parties in Northern Ireland are kept confidential for fear of identifying donors.

But Sinn Fein has demanded that the DUP clarify who funded its "lavish" pro-Brexit advertising campaign.

"The DUP took hugely expensive newspaper ads in a London newspaper which is not circulated in the North and DUP campaign material was also used in other British cities where they do not stand for election," said Sinn Fein Assembly candidate John O'Dowd.

"The DUP need to come clean and clarify who gave them the money and how much they received.

"It is vitally important in the wake of other scandals that the DUP clarify this in order to ensure integrity in the political process," he added.

Mr O'Dowd said there are strict rules around corporate sponsorship and "there can be no place for anyone attempting to get around those rules."

The DUP has refused to release any details about the donations and advertising campaign.

In a statement, a party spokesman said: "The EU referendum was a national campaign and the Democratic Unionist Party was involved in campaigning for a leave vote right across the UK.

"This included members speaking at events in Great Britain and through advertising.

"All advertising was paid for out of party funds. All eligible donations are registered with the Electoral Commission in the required manner.

"The Democratic Unionist Party registered as a permitted participant in the referendum campaign.

"Other political parties, including from Northern Ireland, also registered as participants in the campaign.

"All spending by the Democratic Unionist Party was within the allocated spending cap for permitted participants and was entirely separate from the official Vote Leave Campaign.

"All the necessary returns for the referendum campaign have been submitted to the Electoral Commission and will be published by the Commission in due course."